The invention relates to an underwater installation for use in offshore recovery of oil and gas, particularly at large depths of the ocean.
Oil drilling from floating vessels is a well established technique which can be carried out even at large depths of the ocean. Conventionally, production of oil and gas has taken place by means of fixed installation resting on the seabed. It is difficult to build fixed installations at large depths. Therefore, technique has been developed wherein wellhead and valves belonging thereto are placed on the seabed, and where risers carry hydrocarbons to a vessel at the surface.
Seabed based equipment is to a high degree remote controlled and adapted to the use of a remote control vehicle (a RCV or a ROV) for maintenance, etc. This prior art technique can be used at moderate depths. Using known technique, large depths are difficult to access, and a finished installation will be very expensive.
From Norwegian patent application No. 924962, it is previously known to dispose wellheads on a submerged buoyancy body, from where conductor pipes extend downwardly to wells on the seabed. From the wellhead, hydrocarbons are conducted upwardly to a vessel as previously known. Thus, the buoyancy body serves as an artificial seabed, wherein well completion and production are carried out using prior art technique. If the artificial seabed has a sufficient buoyancy, it may in itself carry a common fixed oil installation.
According to this technique, production wells are drilled in two phases. By means of a floating vessel, a well is drilled to a part of the planned length, e.g. until a 133/8 inches casing is set, whereafter the well is plugged and left. Thereafter, neighbour wells are drilled in the same manner. The last set casings are, at the upper ends thereof, provided with fasteners in order to be extendable upwardly, e.g. in the form of internal or external threads, to be screwed together with another pipe.
A submerged buoyancy body is anchored above the well area and conductor pipes extend from the buoyancy body and downwardly to the wells, where the conductor pipes are attached to the last set casings. The buoyancy body is positioned at a depth so deep that the wave influence becomes insignificant, the body being attached to the seabed by means of tension struts, such as known from floating tension leg platforms.
On the top of the conductor pipe, within the buoyancy body, a blowout valve is mounted as previously known, risers extending upwardly to a drilling vessel. Drilling of the wells may, thus, continue by means of prior art technique, but now from a substantially less depth than the first phase of the drilling, e.g. one hundred and fifty meters. Second drilling phase which is introduced by drilling out the plug set in the first phase may, thus, be carried out by means of simpler equipment than during the first phase.
Finished drilled wells are completed and put in production as previously known.
Use of a submerged buoyancy body forming an artificial seabed makes it possible to recover oil and gas from substantial depths of the ocean. However, the state of the art, such as represented by said NO 924962, falls unnecessarily expensive, substantially due to a very expensive anchoring.